my own issue with using maneuvers to represent this is that a wizard can, in theory, draw in as much extra power as he wants as long as its available. They CAN draw those extra shifts, but if they didn't want to use them they should be able to hold on to them somehow. I wouldn't want to add a roll to a practitioner character on something they can clearly do already.
Unless the maneuver is to HOLD the shifts...?
I don't get what you mean here.
The maneuvers aren't so that you can draw extra shifts, they're there so that you can cast - period.
Lets say I'm prepping for a fight. Before combat, I make 3 maneuvers: Buzzing with Power, Fuelled Up and Extra Reserves.
In a normal combat, I could tag those for a +2 to my spell casting. So this is getting you extra shifts of power.
In a combat where the GM presents the challenge "You cannot draw magic from the area", you have invoke those aspects to be able to cast at all.